1. Field of the Invention
The present invention relates to a device for measuring the co-ordinates of one or several retroflectors applied on an object, in which light from an illumination unit reaches at least one retroreflector via a beam splitter, light reflected back by the retroreflector is separated from the illumination beam path at the beam splitter and the light impinges upon a detector unit with which the impinging of the detected light spot can be determined.
2. State of the Art
A number of processes have been developed for determining optical co-ordinates based on optical marking of the object and determining the position of the optical marking from one or multiple reference points.
Depending on the type of marking these processes can be divided into 2 classes:
A. Processes in which the marking occurs only optically (see A-document DE 4325542). The to-be-measured object does not need to be especially treated. A light beam having a known beam course illuminates the object and is scattered on the surface of the object. The scattering surface of the object represents the marking and is imaged on a position-resolving or image-giving sensor. These processes are distinguished by the necessity of imaging the illuminated surface of the to-be-measured object on the sensor. Measuring is usually not possible in the case of reflecting objects, because radiation falls on the detector only if the detector, the illumination and the surface of the object are in a particular angle position in relation to each other. Retroreflecting objects can principally not be measured, because the illuminating beam path is reflected back within itself .
B. Processes for measuring optically particular points. In these processes, a special marking is applied on or in the to-be-measured object if the object itself does not possess the special optical property.
Common markings are:
1. Luminous markings (see SELCOM company publication: "Precision non-contact measurement is simpler than you think"). PA1 2. Markings that effectively scatter in the used spectral range (relative to the surroundings). PA1 3. Retroreflectors (see European patent EP 0405423B1), KERN company publication SMART310). PA1 Problems due to the restricted focal depth range respectively the necessity of focusing PA1 Falsification of the measurement due to imaging errors, poor contrast respectively reflecting surfaces in the measuring field. PA1 in the case of telecentric illumination, a value for the tilting of the retroreflecting marking, because tilting of the marking leads to a reduction of the active retroreflecting area in the illuminated measuring field, PA1 in the case of diverging or converging illumination, the distance between the illumination/detector unit and the marking can be determined. PA1 1. if the desired measuring range does not start until at a big distance from the illumination/detector unit or PA1 2. if both a near region in front of the illumination/sensor unit and a distance region having better measuring accuracy respectively than in the case of divergent illumination are desired. PA1 1. Improvement of the robustness of the (intensity) measurement by means of: PA1 2. Derivation of further co-ordinates of the object (position of the surface normals respectively axis of the retroreflector in space). PA1 3. Determination of the coordinates of multiple retroreflectors by means of one illumination/detector unit. PA1 2. The different types of illumination occur successively and are detected successively by the same sensor. PA1 3. A combination of the two possibilities can, of course, also be employed and be, in some circumstances, particularly advantageous. PA1 successive illumination with the different types of radiation, PA1 use of one or multiple color-sensitive sensors, PA1 division of the detector beam path into multiple beam paths to the detectors, which respond differently to the individual types of beams, PA1 wavelength-selective beam splitting or PA1 division of the detector beam path into multiple differently filtered part beam paths to the individual detectors.
Measuring according to A and measuring according to the markings 1 and 2 occurs via an image of the luminous respectively illuminated surface of the object or the marking on position-resolving respectively image-giving sensors. For these processes, an imaging optic has to be utilized between the marking and the object. Due to this, the following problems can arise:
In measuring systems according to B.1., a luminous element has to be attached to each interesting point of the object. The element has to illuminate the surrounding space in such a manner that sufficient light falls on the sensor so that the image of the element can be distinctly differentiated from the surroundings in the sensor plane. To do so, the element has to be supplied with energy. This means that either an energy carrier has to be applied, and the size of the element becomes relatively large, or supply cables have to be laid to the object. The costs of applying them to a to-be-measured object are relatively big.
In the processes based on imaging a scattering marking, the contrast between the image of the surrounding and the image of the marking has to be obtained by suitably intensive respectively selective illumination of the measured area. With this type of measuring, in circumstances, considerable measuring errors can occur if there are reflecting or shiny surfaces in the measuring field. Furthermore, strong illumination of the measuring area is often only obtainable with relatively great energy expenditure.
On the other hand, very efficient are processes based on locating a retroreflector, because it practically reflects the entire light impinging on it within itself respectively offset in parallel. Contrary to the above-mentioned processes, they are not based on imaging the surface of the object respectively the surface of the marking, but rather on locating the retroreflector center with a laser beam. The laser beam is aimed at the retroreflector from the measuring position. These processes are distinguished by the position of the reflector being determined from the angle position of the laser. The optical sensors used in these processes are employed solely to minimize or compensate locating errors: if the laser beam hits the center of the retroreflector, the beam is reflected back within itself, otherwise the beam is reflected back offset in parallel. This offsetting in parallel of the beam is measured at the retroreflector with the optical sensors and the locating error is determined therefrom. Absolutely necessary therefor is that the area of the laser beam on the measuring position is smaller than the active area of the retroreflector. Otherwise, the same light spot position in the sensor plane would always be measured and the effect of the retroreflector would only be that this light spot is turned in relation to the incident spot (about the axis of the spot).
These processes are distinguished by the cross section of the laser beam being smaller (usually &lt;10%) than the cross section of the retroreflector. Another distinguishing feature of this measuring process is that determination of the coordinates is by measuring the locating angles, which have to be corresponding precision and have to be determined. The measuring system usually contains a high-precision, and correspondingly expensive respectively high maintenance, mechanics. Another problem is, in some circumstances, aiming the laser beam at the retroreflector, which has to be carried out at least at the start of a measuring series, usually manually. In a large measuring field, this beam "catching" is, in some circumstances, critical and takes considerable time.